Do you need additional support on the corners for the 5 ft. Galvanized Metal Mobilehome Rockface Skirting Panels?

You could anchor a 2X4 or a 4x4 in the ground with concrete and attach the skirting edges to the post.

If support is needed you could anchor a 2X4 or a 4x4 in the ground with concrete and attach the skirting edges to the post, otherwise the panels could just meet at the corners without further support.